<!DOCTYPE html>
<html lang="en">
    <head>
        <meta charset="UTF-8">
        <meta http-equiv="X-UA-Compatible" content="IE=edge">
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
        <title>Document</title>
        <style>
            .box1 {
                width: 200px;
                height: 200px;
                background-color: #bfa;
                float: left;
            }

            .box2 {
                width: 200px;
                height: 200px;
                background-color: orange;
                /* 开启BFC */
                overflow: hidden;
            }

            
            .box3 {
                width: 100px;
                height: 100px;
                background-color: yellow;

                margin-top: 100px;
            }

        </style>
    </head>
    <body>
        <div class="box1">

            <div class="box3"></div>
        </div>

        <!-- <div class="box2"></div> -->

        
    </body>
</html>